1. Field of the Invention
The present invention relates to a small-sized, dual band antenna capable of transmitting and receiving signal waves of two type frequency bands and adapted for easier assembly with a car-mounted communication device.
2. Description of the Related Art
Conventionally, as a dual band antenna suitable for miniaturization, an inverted F type antenna that can resonate with two kinds of frequencies, i.e., high and low frequencies by the action of notch portions provided on a radiating conductor plate (refer to Patent Document 1), has been proposed.
FIG. 5 illustrates a conventional example. In FIG. 5, an inverted F type dual band antenna 1 includes a radiating conductor plate 2 with a rectangular notch portion 4. The radiating conductor plate 2 has an L-shaped conductor piece 2a resonating at a first frequency f1 and a rectangular conductor piece 2b resonating at a second frequency f2 that is higher than the first frequency f1. Extending from one lateral edge of the radiating conductor plate 2 is a conductive connector plate 3, which is mounted upright on a grounding conductor plate 5 so that the radiating conductor plate 2 can be electrically connected to the grounding conductor plate 5. The entire surface of the radiating conductor plate 2 is spaced apart from the grounding conductor plate 5 at a predetermined gap (the height of the conductive connector plate 3). A power-supplying pin 6 is soldered to a given position on the radiating conductor plate 2. The power-supplying pin 6 is connected to an antenna circuit not shown in the figure, without making contact with the grounding conductor plate 5.
In the conventional dual band antenna 1 of the construction as stated above, the L-shaped conductor piece 2a has a length along the extending direction thereof equal to approximately a quarter of the resonance length λ1 corresponding to the first frequency f1, and the rectangular conductor piece 2b, which extends shorter than the L-shaped conductor piece 2a, has a length equal to approximately a quarter of the resonance length λ2 (λ2<λ1) corresponding to the second frequency f2. For this reason, this assures that, when a predetermined high frequency electrical power is supplied to the radiating conductor plate 2 through the power-supplying pin 6, the conductor pieces 2a and 2b can resonate at mutually different frequencies, thereby making it possible to transmit and receive signal waves of two kinds of frequency bands, i.e., high and low frequency bands.
[Patent Document 1]
Japanese Unexamined Patent Application Publication No. 10-93332 (pages 2 to 3, FIG. 1)
In the conventional dual band antenna illustrated in FIG. 5, the electrical waves emitted from the L-shaped conductor piece 2a at the time of resonating at the first frequency f1 have directivity as shown in FIG. 6A which assures increased gain in the horizontal direction as well as in the upward direction. However, electrical waves emitted from the rectangular conductor piece 2b at the time of resonating at the second frequency f2, which is higher than the first frequency, have directivity biased upwards, as shown in FIG. 6B, which leads to reduced gain in the horizontal direction. Because a car-mounted communication device often receives and transmits horizontally advancing signal waves, employing the conventional dual band antenna 1 as a car-mounted communication antenna may result in the electrical waves of the second frequency f2 being used incompletely. Particularly, sensitivity will be dramatically reduced in cases where the resonance frequency of the rectangular conductor piece 2b deviates from the predetermined second frequency f2. In practical application, resonance frequency often deviates from a predetermined value under the influence of an antenna mounting bracket, etc., and it is difficult to correct such deviation in the conventional dual band antenna 1.